Mendel and the Gene Idea

Chapter 14

• Gregor Mendel - monk - studied pea plants, looked at traits.

• Pea plants many varieties with distinct heritable features (characters) with different variants (traits).

• Pea plants self-fertilize; Mendel cross-fertilized to study traits.

• Mendel cross-pollinated (hybridize) 2 contrasting, true-breeding pea varieties.

• True-breeding parents - P generation; hybrid offspring - F1 generation.

• F1 hybrids then self-pollinate to produce F2 generation.

http://nitro.biosci.arizona.edu/courses/EEB320-2005/Lecture02/pics/pea.jpeg

• Thought genes blended - purple flower crossed with white flower result would be light purple flowers.

• All the flowers purple.

• When flowers self-fertilized, white flower reappeared in next generation.

• Ratio of purple to white in F2

generation was 3:1.

• Developed hypothesis to explain process.

• 1Alternative versions of genes (alleles) account for variations in inherited characters.

• 2For each character organism inherits 2 alleles, 1 from each parent.

• Alleles can be same or different.

• 32 alleles differ - 1 (dominant allele) fully expressed in organism.

• Other (recessive allele) no noticeable effect on organism’s appearance.

• 42 alleles for each character segregate (separate) during gamete production.

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• Found that alleles not linked due to inheritance patterns.

• Independent assortment of each pair of alleles during gamete formation - law of independent assortment.

• Therefore, Mendel had three different laws.

• The law of dominance and recessiveness states that one gene is dominant over the more recessive gene.

• The law of segregation states that alleles separate during meiosis.

• The law of independent assortment states that alleles organize in the gametes regardless of other alleles.

• An organism with two identical alleles for a character is homozygous for that character.

• Organisms with two different alleles for a character is heterozygous for that character.

• A description of an organism’s traits is its phenotype.

• A description of its genetic makeup is its genotype.

• A Punnett square predicts the results of a genetic cross between individuals of known genotype.

• A testcross, breeding a homozygous recessive with dominant phenotype, but unknown geneotype, can determine the identity of the unknown allele.

• Mendel’s experiments focused on monohybrid crosses meaning that he looked at only one trait at a time.

• Later on he started looking at dihybrid crosses involving probabilities of two different traits.

• Mendel’s ideas are based on probability.

• If you were to toss a coin 4 times, the coin has a ½ chance of coming up heads every time.

• Each toss is independent of the one done before.

• The probability of it coming up heads all four times is: ½ * ½ * ½ * ½ = 1/8.

• This is known as the rule of multiplication.

12.2

• There is also a law of addition that determines the chances of an event happening in different ways.

• For example, there are two ways that F1 gametes can combine to form a heterozygote.

• The dominant allele could come from the sperm and the recessive from the ovum (probability = 1/4).

• Or, the dominant allele could come from the ovum and the recessive from the sperm (probability = 1/4).

• The probability of a heterozygote is 1/4 + 1/4 = 1/2.

• Incomplete dominance can also occur in offspring.

• In incomplete dominance, heterozygotes have a completely different phenotype than homozygotes.

• This happens in snapdragons.• Homozygous recessive flowers are

white; homozygous dominant flowers are red; heterozygotes are pink.

• Another inheritance pattern is codominance in which two alleles affect the phenotype in separate, distinguishable ways.

• An example of this is blood type.• If you inherit an A allele and a B

allele, your blood type will be AB; if it is AA or AO, your blood type will be A.

• This means that A is dominant to O, B is dominant to O, but A is codominant to B.

• Blood type is important because type A has anti-B antibodies.

• If exposed to B blood, it will clump together causing a transfusion reaction.

• People with blood type O have both antibodies and therefore can donate to any other blood type.

• On the other hand, AB has neither antibodies and therefore can receive from any blood type.

• Dominant genes do not mean that they are more popular in a given population.

• Also, most genes do not control only one trait but are pleiotropic, affecting more than one phenotypic character.

• In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus.

• In mice, one gene determines whether or not there will be a coat color.

• If that gene is turned off, the mouse will be white; if it is turned on, another locus will determine what the color is (brown or black).

• Quantitative characters vary in a population along a continuum.

• This is because of polygenic inheritance which is when more than one gene controls a single trait.

• An example of this is skin color which is controlled by at least three different genes and is responsible for the variety of skin colors.

• Phenotype also depends on environment.

• For humans, nutrition influences height, exercise alters build, sun-tanning darkens the skin, and experience improves performance on intelligence tests.

• Genetic experiments cannot ethically be performed on humans, so geneticists use pedigrees to look at traits found in families.

• A family tree is then created showing the absence or presence of a specific trait to determine how it is passed.

• Ethnicity plays a role in genetic disease patterns.

• For example, sickle-cell anemia is found predominately in African-Americans.

• This disease causes the red blood cells to be sickle shaped instead of the normal disk shape causing the cells to get stuck in the vessels.

• Cystic fibrosis affects mostly Caucasians.

• Cystic fibrosis is a multi-system disease that causes mucous to build up in various organs, especially the lungs.

• Tay-Sachs affects people of Jewish descent.

• Tay-Sachs affects the brains of small children, ultimately causing the death of the child prior to 5 years old.

• Some genetic diseases, such as dwarfism, are dominant diseases.

• This means that a child has a 50% chance of inheriting the disease because one of the parents has the disease.

• Huntington’s disease is also a dominant disease that affects the nervous system.

• Most dominant diseases are not lethal (Huntington’s disease is).

• Genetic counseling is a field of study that works with people that have a history of genetic disease in the family.

• A child with a recessive disease can be born to phenotypically normal parents.

• There are several tests that can be performed to determine a couple’s risk.

• One technique is amniocentesis.• Cells are extracted from the amniotic

fluid surrounding the fetus and then analyzed to search for potential problems in a technique called karyotyping.

• Karyotyping is essentially mapping out the chromosomes of an individual.

• A second technique, chorionic villus sampling (CVS) can allow faster karyotyping and extracts a sample of fetal tissue from the chorionic villi of the placenta.

• A more routine test is an ultrasound which detects only physical abnormalities that are present.